POLYSACCHARIDE AEROGEL REINFORCED PLASTICS

Description

[0001] The present invention relates to plastics reinforced with polysaccharide aerogel and to processes for their production. In particular, plastics reinforced with polysaccharide aerogel fibers in the form of three-dimensional networks are provided, wherein the polysaccharide aerogel and the plastic are positively and non-positively bonded, and the composite body is free of air inclusions. The polysaccharide is preferably cellulose.

[0002] Fiber-reinforced plastics (FRP) consist of at least two components, the combination of which ideally exhibits the best properties of both individual components. Conventional fiber materials include glass or carbon fibers as reinforcing components, as well as a matrix material encasing the fibers, usually made of plastic resins, resulting in fiber-reinforced plastics such as glass fiber reinforced plastics (GRP) and carbon fiber reinforced plastics (CFRP).

[0003] With the growing interest in renewable raw materials, natural fibers are increasingly becoming the focus of scientific research not only as fillers but also as reinforcing components. A significant group of natural fibers is polysaccharides, the most important of which is cellulose.

[0004] WO 2015 / 055338 A1 describes the wetting of fiber mats with aerogel fibers. Pirchner, N et al., "Reinforcement of bacterial cellulose aerogels with biocompatible polymer," uses bacterial cellulose, whose solid-to-void ratio clearly does not fall under the term aerogel. Tuukka, Nissilä: "Oriented nanofiber templates for resin impregnation of cellulose nanocomposites", December 31, 2016 (2016-12-31), pages 1-108, XP055561727, available online at http: / / jultika.oulu.fi / files / nbnfioulu-201701121049.pdf and Zhai, Tianlang et al.: "Poly(vinyl alcohol) / Cellulose nanofibrill hybrid aerogels with an aligned microtubular prous structure and their composites with polydimethylsiloxane" Applied Material & Interfaces, Vol. 7, No. 13, March 30, 2015 (2015-03-30), pages 7436-7444, XP093029155 Describe the infiltration of nano-cellulose fibers with Polymers. This creates directional, unidirectional structures.

[0005] There is increasing scientific focus on the production of cellulose-based nanocomposites. Cellulose is a promising renewable raw material with high strength (up to 150 MPa) and a relatively low density (approximately 1.5 g / cm³).

[0006] Assessing the strength and stiffness of these materials, considering their low density, composites of polysaccharides such as cellulose with a suitable matrix would be competitive with conventional composites in terms of their specific strength / stiffness. Some comparative values ​​are listed in Table 1.

[0007] For several years, novel nanocomposites based on cellulose and thermoplastic polymer systems have been described in the literature, for example by Dufresne (Dufresne, Nanocellulose: a new ageless bionanomaterial, Materials Today, 16, 6, (2013) 220-227) and Oksman (Oksman et al., Review of the recent developments in cellulose nanocomposite Processing, Composites: Part A 83 (2016) 2-18).

[0008] Temperature is a crucial factor in the processing of thermoplastics, as high temperatures often lead to degradation or at least impairment of the original properties of the cellulose and thus to inferior composites, as Lee (K.-Y. Lee et al., On the use of nanocellulose as reinforcement in polymer matrix composites, Composites Science and Technology 105 (2014) 15-27) and also the Handbook of Nanocellulose and Cellulose Nanocomposites (Kargarzadeh (Editor), Ahmad (Editor), Thomas (Editor), Dufresne (Editor), Handbook of Nanocellulose and Cellulose Nanocomposites (2017)).

[0009] The incorporation into thermosetting plastics is also described, for example, by Sabo (Sabo et al., Characterization and Processing of Nanocellulose Thermosetting Composites, book chapter in, "Technological advancement in polymer nanocomposites of cellulose nanoparticle: processing, performance and applications, (2015) 265-287) and Ramamoorthy (Ramamoorthy, Properties and performance of regenerated Cellulose Thermoset Biocomposites, Phd-Thesis, University of Boras, Sweden (2015)). To date, this has mainly taken place in the form of powders. However, due to the density difference between fiber and thermosetting matrix material, the problem arises that sedimentation of the fiber materials often occurs. The approaches pursued to date in the state of the art therefore do not lead to the production of homogeneous composite bodies for either thermoplastics or thermosets.

[0010] A study by Pircher (Pircher et al., Reinforcement of bacterial cellulose aerogels with biocompatible polymers, Carbohydrate Polymers, 111 (2014) 505-513) describes the incorporation of bacterial cellulose aerogels into the biocompatible thermoplastic polymers of polylactic acid (PLA), polycaprolactone (PCL), cellulose acetate, and polymethyl methacrylate (PMMA). This study uses dilute solutions, with the goal of subsequently removing the cellulosic substrate to obtain an inverse thermoplastic gel.

[0011] This is achieved by conducting pore fluid exchange in the wet state of the bacterial cellulose, followed by supercritical drying. This involves using pre-prepared polymers and then removing the cellulose with a solvent. This results in the desired inverse thermoplastic gel.

[0012] Fiber-reinforced plastics present several difficulties, especially during the manufacturing process. Manufacturing using resin transfer molding (RTM) or resin infusion molding (RIM) always carries the risk of incomplete filling of the pore spaces, which can lead to air inclusions and predetermined breaking points in the final component. When incorporating cellulose in the form of nano- or microcrystalline cellulose, Dufresne (Dufresne, "Nanocomposites based on natural fibers," Chapter 6 in Natural Fiber Reinforced Polymer Composites: From Macro to Nanoscale eds. Sabu, Pothan (2009) 142-165) points to the problem of sedimentation, which leads to an inhomogeneous property distribution.

[0013] Fiber reinforcement in plastics can be provided in various forms, for example, as short fiber reinforcement or continuous fiber reinforcement. However, none of these reinforcements forms a cohesive fiber system that acts in all three spatial directions.

[0014] Glass fiber reinforced plastics (GRP) and carbon fiber reinforced plastics (CFRP), in the case of continuous fibers, are always used in non-crimp or woven fabrics and exhibit outstanding properties only in the direction of the fibers. If the load axis is perpendicular to the fiber direction, the forces are primarily absorbed by the matrix. An improvement in this case can be achieved by stacking layers at an angle.

[0015] Both glass and carbon fibers require pretreatment or the application of a bonding agent, which improves or even enables the compatibility between the fiber and the matrix. Only this pretreatment improves the force transfer from the matrix to the highly modulus fiber material. Chemical modifications are also occasionally necessary for nanocrystalline cellulose to ensure better compatibility between fiber and matrix, as documented by Dufresne (Dufresne, Nanocomposites based on natural fibers, Chapter 6 in Natural Fiber Reinforced Polymer Composites: From Macro to Nanoscale eds. Sabu, Pothan (2009) 142-165).

[0016] Compared to the prior art presented above, the present invention is therefore based on the object of providing stable composite bodies that have no or only minimal air inclusions (nanoscale, less than 5 nm in diameter), whose properties are as homogeneous and direction-independent (anisotropic) as possible, and which are also based on renewable raw materials. In particular, such composites are to be made from natural fibers and thermoplastic or thermosetting plastics.

[0017] Any polysaccharide in aerogel form can be used as a polysaccharide aerogel. Amylose, (amylo)pectin, pectins, chitin, and / or cellulose can be used in particular. Cellulose aerogel is preferred. Thermosets or thermoplastics can be used as plastics.

[0018] The polysaccharide aerogel is preferably in the form of open-pore fibers, macroscopically in particular in the form of two-dimensional textile structures such as nonwovens and / or woven fabrics and / or in the form of a nanoscale three-dimensional network. Such a three-dimensional network of cellulose aerogel is particularly preferred. The fibers preferably have diameters of 5 to 25 nm. Suitable fibers are described, for example, in DE 10 2006 049 179 1.

[0019] Compared to the prior art, the composite body according to the invention is distinguished in particular by the fact that the fiber component of the polysaccharide-based composite material is in the form of an aerogel. The material and force-fit bond between the reinforcing fiber and the matrix ensures the stability of the composite body.

[0020] Surprisingly, it has been shown that the mandatory application of a bonding agent for GRP and CFRP can be avoided because polysaccharides contain various functional groups capable of reacting with the plastic matrix material. This creates the possibility of forming a chemical bond between the fiber and matrix material, which can ensure force transmission. The chemical modification required, for example, for nanocrystalline cellulose wipers is also not absolutely necessary, especially when using cellulose aerogel.

[0021] Instead of staggered fiber layers, aerogels can be used as a random three-dimensional network, which exhibits identical properties in all three spatial directions. However, when ordered textile fabrics such as woven fabrics are used, the composite body absorbs forces in a directional manner.

[0022] Surprisingly, the use of a polysaccharide with an aerogel structure also prevents fiber sedimentation, thus preventing an inhomogeneous property distribution. The reinforcing component (e.g., cellulose aerogel) is already present in its final morphology in the form of a polysaccharide aerogel and does not dissolve in the matrix. The nanoscale fibers are located in a solid three-dimensional network and cannot sink as individual fibers in the matrix, which is liquid during production, despite the density difference.

[0023] The three-dimensional network of the polysaccharide aerogel is firmly bonded and frictionally connected, as a chemical bond forms where two or more nanofibrils touch. Therefore, slippage between two fibers is impossible.

[0024] Furthermore, it has surprisingly been shown that cellulose aerogels in particular are capable of transporting a polymerization liquid with sufficient viscosity into the pore space through capillary forces during the composite manufacturing process. This reduces the number of predetermined breaking points and thus also waste. An important distinguishing criterion from state-of-the-art methods is the utilization of capillary forces resulting from the small pore sizes (20 to 200 nm) in cellulose aerogels. Due to this fine pore network, the cellulose aerogel is capable of absorbing a polymerization liquid even without applying negative pressure, using only the resulting capillary forces, thus creating a bubble-free composite.In particular, using the ultrasound methods commonly used to detect gas inclusions in fiber-reinforced composite bodies, it has been proven that no gas inclusions are found in the composite bodies according to the invention.

[0025] The three-dimensional network, coupled with the thermal properties of the polysaccharide, especially cellulose, can also enable relatively simple degradation of the composite body according to the invention at the end of its product life. When thermal decomposition of the polysaccharide-plastic composite begins, the fiber material provides oxygen, which can result in simple degradation with fewer harmful degradation products. In particular, simple thermal degradation of cellulose-plastic composites with fewer harmful degradation products is possible.

[0026] In a preferred embodiment, the composite body according to the invention is free of air inclusions and / or solvents. Freedom from air inclusions is achieved by completely filling the pore spaces of the polysaccharide aerogel with plastic material. This prevents the formation of predetermined breaking points and thus improves the stability of the composite body according to the invention.

[0027] In one embodiment of the composite body according to the invention, the plastic used is a thermoplastic. In particular, acrylates, polyamides, lactides, polystyrenes, and / or polycarbonates can be used as thermoplastics.

[0028] Suitable acrylates include, for example, polymethyl methacrylate and polyacrylic acid, but the invention is not limited to these. Suitable polyamides include, for example, PA-6, PA-6,6, PA-8, PA-12, and the like, but the invention is not limited to these. Suitable lactides include, for example, polylactic acid, but are not limited to these.

[0029] Surprisingly, it has been shown that, particularly when acrylates such as polymethyl methacrylate are used as the thermoplastic component and cellulose as the aerogel fiber material, a composite body that is optically transparent in the liquid phase and translucent in the solid phase can be obtained. Due to the fine porosity at low fiber contents of the aerogel network, the transparent properties of the plastic used are retained. In a preferred embodiment, the composite body according to the invention can be transparent or translucent, but at elevated saccharide concentrations (more than 4% by weight) it is opaque to whitish.

[0030] In an alternative embodiment of the composite body according to the invention, the plastic used is a thermoset.

[0031] In particular, epoxy resins, phenol-formaldehyde resins, melamine-formaldehyde resins, saturated and / or unsaturated polyester resins, cyanoacrylates and / or polyurethanes can be used as thermosetting plastics.

[0032] The problem underlying the invention is solved by a method according to claim 1.

[0033] The method according to the invention is characterized in particular by infiltrating a provided polysaccharide aerogel, in particular a cellulose aerogel, with a polymerization liquid, and subsequently forming the composite body by polymerization, polyaddition, or polycondensation of this liquid. This allows the polymerization liquid to penetrate the aerogel network in a material-to-material manner. In the composites of polysaccharides and plastics known from the prior art, in contrast, prefabricated polymer is combined with the polysaccharide in a solvent, and the solvent is subsequently removed, thus preventing a force-to-material bond. Surprisingly, it has been shown that such a bond can be achieved using the method according to the invention.

[0034] The polymerization liquid can be, for example, liquid polymerizable monomers or precondensed oligomers, optionally diluted in a suitable solvent. The polymerization liquid can be suitable and intended for the synthesis of a thermoplastic or thermosetting plastic.

[0035] Cationic, anionic and / or radical processes using radical initiators and / or IR, UV initiators can be used as polymerization processes.

[0036] During the process, the temperature is preferably kept below 250°C. More preferably, the temperature is below 180°C. Most preferably, the temperature is below 150°C.

[0037] In a preferred embodiment of the process according to the invention, the polysaccharide aerogel is provided in the form of fibers, in particular in the form of two-dimensional textile structures such as nonwovens and / or woven fabrics and / or in the form of a three-dimensional network. Particularly preferably, a cellulose aerogel is provided in the form of a three-dimensional network. The fibers preferably have diameters in a range of 5 to 25 nm.

[0038] The polymerization liquid is preferably free of solvents. In a preferred embodiment, the polymerization liquid comprises low-viscosity monomers, oligomers, and / or precondensates. The polymerization liquid is particularly preferably free of polymers. Preferred oligomers comprise more than 10 monomer units and, for example, up to 30 monomer units. The polymerization liquid preferably has a viscosity of less than 100 Pa s. The viscosity is particularly preferably less than 10 Pa s, most preferably less than 1 Pa s.

[0039] The following describes the process in more detail using cellulose aerogel as a polysaccharide aerogel as an example. However, the process according to the invention is not limited to this description or to such aerogels. The procedure may differ depending on whether thermosets or thermoplastics are to be used as the plastic.

[0040] The process according to the invention can be carried out for thermosets, for example, as described below. Dry cellulose aerogels of all forms (sheets, fibers, etc.) are placed in a mold and impregnated with a precondensed resin system (phenol-, epoxy-, polyester-, polyesterurethane-, or cyanoacrylate ester-based resin system, diluted with suitable solvents or in pure form). Applicable processes for infiltrating cellulose aerogels include resin infusion molding (RIM) and resin transfer molding (RTM), which are also used for CFRP / GFRP. Due to the open-pore structure of the cellulose aerogel, capillary infiltration is also possible, which requires no additional aids or, at most, an increase in temperature, which serves to accelerate the process.In RIM, the use of various membrane materials, flow aids, and films is recommended to enable complete infiltration. Provided the viscosity is sufficiently low (especially less than 100 Pa s), the entire pore space of the aerogel is filled with the matrix system. If necessary, an increase in temperature may be necessary to promote the flow behavior of the aerogel. The use of vacuum-assisted methods (RIM and the films, flow aids, etc. used with it) as well as an increase in temperature shortens the production time for bubble-free infiltrated aerogels. Resin systems used must have long pot / open times (even at elevated temperatures) to ensure complete infiltration. Preferred pot lives are between 30 and 300 minutes. The lower the viscosity, the shorter the pot life can be.The shortened pot life at elevated temperatures and the presence of solvents that evaporate when negative pressure is applied must be taken into account. After gelation, curing, and any necessary post-curing, a composite body reinforced with polysaccharide aerogel, particularly cellulose aerogel, is obtained. The process according to the invention can be carried out at temperatures up to 180 °C without causing any significant changes in the nanofiber material.

[0041] The composite bodies are characterized by a low density and a low fiber content compared to glass or carbon fibers. Figure 1A comparison of the densities of current materials with inventive cellulose aerogel-reinforced thermosets is shown depending on the commonly used fiber content (GFRP: glass fiber reinforced plastics, CFRP: carbon fiber reinforced plastics, CA: cellulose aerogel, CARP: inventive cellulose aerogel fiber reinforced plastics).

[0042] The properties of this composite body according to the invention differ significantly from those of the individual components. Cellulose aerogels themselves exhibit relatively poor mechanical properties, which results from the fact that they consist predominantly of air. With a porosity between 65% and 98%, very low solids contents are used. The elastic moduli of cellulose aerogels (1 to 10 wt%) produced from aqueous salt hydrate melts range from 10 to 280 MPa and show a strong dependence on the solids content.

[0043] Figure 2 shows the mechanical properties of two thermoset CARPs according to the invention. The upper half shows the compression moduli from compression testing with a phenolic resin matrix, while the lower half shows the elastic moduli from tensile testing with an epoxy resin matrix.

[0044] Further studies have shown that, despite the partially hygroscopic polysaccharides contained, there is no change in water absorption compared to the pure matrix system.

[0045] The process according to the invention can be carried out for thermoplastics, for example, as described below. The fine-pored aerogel network can be filled with sufficiently low-viscosity media (less than 100 Pa s). Dry cellulose aerogels of all forms (sheets, fibers, etc.) are impregnated with monomers of thermoplastic polymers (amides, acrylates, vinyl compounds, esters, lactates, carbonates, etheretherketones, etc.). Within a very short time, the monomers / polymerization liquid are absorbed into the aerogel pore space by capillary forces. After subsequent polymerization in the pore space, a thermoplastic aerogel composite body is obtained. The introduction of the cohesive and force-fit network results in significant changes in the properties of the respective thermoplastics. For example, the glass transition temperature of the thermoplastics used is significantly increased.For example, cellulose aerogel-polystyrene composites with a cellulose content of 3% by weight show a double-digit percentage increase in the glass transition temperature. Similar behavior within a comparable range can also be observed with other thermoplastics. Glass transition temperature shifts of up to 40% have been observed. Infiltration with clear, transparent thermoplastics also imparts a high degree of transparency to the composite (in the wet, but not fully cured, state). Temperature-induced reactions (e.g., the decomposition of initiators to radical initiators) can be carried out at temperatures up to 180 °C.

[0046] The object of the invention is achieved by a composite body comprising polysaccharide aerogel, in particular cellulose aerogel, and at least one plastic, obtainable by the method according to the invention described above. The plastic can be, for example, a thermoset or a thermoplastic.

[0047] The object of the invention is achieved by using polysaccharide aerogel, in particular cellulose aerogel, to reinforce plastics in a composite body. The reinforced plastics can be, for example, thermoplastics or thermosets. Example 1

[0048] A cellulose aerogel with 5% cellulose by weight is used as the fiber reinforcement material. Before infiltration, the residual moisture in the aerogel is removed by drying in a vacuum drying oven at 120 °C and 10 mbar for at least 3 hours. The completely dry aerogel is covered with a phenolic resin in a pre-tempered state (approximately 90 °C) and placed in an oven. A phenol-formaldehyde resin (pre-condensed, diluted with 2-propanol) with a viscosity of 2 Pa s is used for infiltration. Infiltration occurs exclusively through capillary forces. Working at an elevated temperature of 60 to 80 °C allows for a reduction in the resin viscosity, which accelerates infiltration. Within a few hours, the entire pore volume of the aerogel is completely filled with the resin. This green compact is transparent and shows no air inclusions that are visible to the naked eye or detectable by ultrasound.Since the resin system has not yet gelled, this green body can now be removed and molded into the desired final shape. It then undergoes complete curing in an oven at 100 °C. If necessary, a vacuum can be used to remove excess matrix material. To achieve optimal matrix properties, a subsequent curing step is usually performed. Example 2

[0049] A cellulose aerogel with 5% cellulose by weight is used as the fiber reinforcement material. Before infiltration, the residual moisture in the aerogel is removed by drying in a vacuum drying oven at 120 °C and 10 mbar for at least 5 hours.

[0050] After cooling to room temperature, the aerogel can be further processed. This example involves the radical polymerization of styrene in the bulk. Styrene contains commercially available stabilizers. These can either be washed out or compensated for by an excess of initiator adjusted to the amount of stabilizer.

[0051] Styrene is mixed with at least 0.1% of a suitable initiator. Azo compounds such as azobis(isobutyronitrile) (AIBN) or peroxides such as (di-)benzoyl peroxide (DBPO) can be used as radical initiators. The dry aerogel is immersed in the solution at room temperature under a protective gas. Within a few minutes, the entire pore volume of the aerogel is completely filled with the monomer solution. Gas bubbles can be observed rising from the aerogel. The aerogel becomes transparent and shows no air pockets that are visible to the naked eye or detectable under a light microscope. Once the filled aerogel is completely transparent, it can be heated (preferably under a protective gas) to a temperature of 70 to 110 °C to initiate the initiator decomposition into radical initiators.After polymerization, a cellulose-polystyrene composite is obtained, which appears opaque to whitish at a cellulose content of 5% by weight.

Claims

1. Method for producing a composite body comprising a polysaccharide aerogel, especially a cellulose aerogel, and at least one plastic material, in which said plastic material and said polysaccharide aerogel are bonded together by both adhesive and frictional connection, wherein said method comprises the following steps: i) providing a polysaccharide aerogel, especially cellulose aerogel, in a molded body, ii) infiltrating said polysaccharide aerogel with a polymerization liquid of said plastic material, and iii) curing said polymerization liquid to obtain the composite body.

2. Method according to claim 1, characterized in that said polysaccharide aerogel is provided in the form of fibers, especially in the form of two-dimensional textile structures sheet fabrics, such as non-wovens and / or woven fabrics and / or in the form of a three-dimensional network.

3. Method according to claim 1 or 2, characterized in that said polymerization liquid is free of solvents.

4. Method according to any of claims 1 to 2, characterized in that said polymerization liquid comrpises low viscosity monomers, oligomers and / or precondensates, especially is free of polymers.

5. A composite body, including a polysaccharide aerogel, especially a cellulose aerogel, and at least one plastic material, obtainable by a method according to any of claims 1 to 4.

6. Composite body according to claim 5, characterized in that it includes a polysaccharide aerogel, especially a cellulose aerogel, in the form of fibers, especially in the form two-dimensional textile structures of sheet fabrics, such as non-wovens and / or woven fabrics and / or in the form of a three-dimensional network.

7. Composite body according to claim 5 or 6, characterized in that said composite body is free of air inclusions and / or solvents.

8. Composite body according to any of claims 5 to 7, characterized in that said plastic material is a thermoplastic material.

9. Composite body according to claim 8, characterized in that said plastic material is a polyacrylate, polyamide, polylactide, polystyrene, and / or polycarbonate.

10. Composite body according to any of claims 5 to 9, characterized in that said composite body is optically transparent, translucent, and / or opaqued to whitish.

11. Composite body according to claim 5 or 6, characterized in that said plastic material is a thermosetting material.

12. Composite body according to claim 11, characterized in that said plastic material is epoxy resins, phenol-formaldehyde resins, melamine-formaldehyde resins, saturated and / or unsaturated polyester resins, cyanoacrylates, and / or polyurethanes.

13. Use of a polysaccharide aerogel, especially cellulose aerogel, in methods according to claims 1-4 for reinforcing plastic materials in a composite body.